JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES, VOL. 118, 1–11, doi:10.1002/jgrd.50595, 2013
The meteorology of negative cloud-to-ground lightning strokes
with large charge moment changes: Implications
for negative sprites
Timothy J. Lang,1,2 Steven A. Cummer,3 Steven A. Rutledge,1 and Walter A. Lyons 4
Received 28 December 2012; revised 18 June 2013; accepted 24 June 2013.
[1] This study examined the meteorological characteristics of precipitation systems that
produced 38 “sprite-class” negative cloud-to-ground (CG) strokes (i.e., peak currents in
excess of 100 kA and charge moment changes in excess of 800 C km) as well as those that
produced three confirmed negative sprites on 23 different days during 2009–2011. Within
15 km of the negative sprite-parent/class stroke, the median characteristics for these systems
were to produce negative CGs as 69.2% of all CGs, and for the 30 dBZ radar reflectivity
contour to reach on average 14.2 km above mean sea level (MSL), during a 25 min period
encompassing the occurrence of the stroke. The median contiguous area of 30 dBZ
composite radar echo (i.e., maximum value in the vertical column) for these systems was
6.73 × 103 km2. All but three of the discharges occurred in intense multicellular convection,
with 30 dBZ exceeding 10 km MSL in altitude, while the others occurred in the stratiform
regions of mesoscale convective systems. All but six of the systems produced greater than
50% negative CG lightning, though flash rates tended to be low near the stroke (1–2 min 1
on average). The results suggest that negative sprite-parent/class lightning typically occurs
in precipitation systems of similar size and intensity as those that produce positive sprites,
but not necessarily the same systems, and the negative lightning normally strikes ground in
the convection rather than the stratiform precipitation. However, upper-level positive charge
in the convection may play an important role in sprite-class/parent lightning of
either polarity.
Citation: Lang, T. J., S. A. Cummer, S. A. Rutledge, and W. A. Lyons (2013), The meteorology of negative cloud-toground lightning strokes with large charge moment changes: Implications for negative sprites, J. Geophys. Res. Atmos., 118,
doi:10.1002/jgrd.50595.
1.
Soula et al., 2009; Lu et al., 2012; Li et al., 2012]. By
contrast, a single precipitation system can produce more than
10 times that number of observed positive sprites in just a few
hours [e.g., Lang et al., 2010].
[3] The scientifically accepted theory of sprite production is
polarity agnostic [Wilson, 1925], so the relative rarity of negative sprites in relation to positive sprites has been a topic of interest in the atmospheric electricity community. Research has
progressed along two main lines of inquiry. One has been
demonstrating that while negative sprites may be rare, other
transient luminous events (TLEs) like negative halos are not
and indeed may be as common or more common than their
positive counterparts [Bering et al., 2004; Frey et al., 2007;
Williams et al., 2007, 2012; Newsome and Inan, 2010]. The
other line of inquiry has demonstrated that negative cloud-toground (CG) lightning with large charge moment changes
(CMCs) is much rarer than its positive-CG counterpart.
Given the primacy of CMC in determining the likelihood of
sprite production (typically hundreds of C km are needed to
initiate sprites) [Huang et al., 1999; Hu et al., 2002;
Cummer and Lyons, 2005; Lyons et al., 2009; Qin et al.,
2012], it then follows that negative sprites should be much
rarer since only a small number of negative CGs have the
requisite characteristics for producing sprites [Cummer and
Introduction
[2] While the meteorology of positive sprites (i.e., sprites
associated with positive cloud-to-ground lightning) is
relatively well known, at least in a gross sense [Lyons,
1996, 2006; Williams and Yair, 2006; Lyons et al., 2009],
the meteorology of negative sprites (i.e., sprites associated
with negative cloud-to-ground lightning) is largely
unexplored. This is because there are so few observations
of negative sprites. To date, there have been only a small
number of peer-reviewed studies documenting the occurrence of approximately 10 confirmed negative sprites
[Barrington-Leigh et al., 1999, 2001; Taylor et al., 2008;
1
Department of Atmospheric Science, Colorado State University, Fort
Collins, Colorado, USA.
2
Now at NASA Marshall Space Flight Center (ZP11), Huntsville, USA.
3
Department of Electrical and Computer Engineering, Duke University,
Durham, North Carolina, USA.
4
FMA Research, Inc., Fort Collins, Colorado, USA.
Corresponding author: T. J. Lang, NASA Marshall Space Flight Center
(ZP11), Huntsville, AL 35812, USA. (timothy.j.lang@nasa.gov)
©2013. American Geophysical Union. All Rights Reserved.
2169-897X/13/10.1002/jgrd.50595
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LANG ET AL.: LARGE CHARGE MOMENT NEGATIVE LIGHTNING
they mainly occur over convective regions of large mesoscale precipitation systems and that these convective regions
contain normal polarity tripoles and thus produce predominantly negative CG lightning. The purpose of this paper is
to test this hypothesis and thus shed additional light on the
nature of precipitation systems that produce negative sprites,
or powerful negative CGs that likely created a sprite, even
though camera observations were not available.
Lyons, 2005; Williams et al., 2007; Cummer et al., 2013].
Moreover, the minimum CMC threshold for initiating negative sprites may be ~50% greater than that for positive sprites
[Qin et al., 2012].
[4] Positive sprites are normally associated with positive
CGs in the stratiform regions of mesoscale convective
systems (MCSs) [Boccippio et al., 1995; Lyons, 1996,
2006; Lyons et al., 2003; Williams and Yair, 2006]. For
example, Lyons [1996, 2006] has found that positive sprites
tend to occur after a mesoscale system reaches 20,000–
25,000 km2 in total area and contains both active convection
as well as a large region of stratiform precipitation. The
sprite-parent discharges either initiate within the convective
region and propagate out into the stratiform region before
coming to ground, or initiate in situ within the stratiform
region before coming to ground [Lang et al., 2010]. The large
CMCs produced by these CGs are primarily the result of
discharging the laterally extensive regions of positive charge
that exist within the stratiform regions of MCSs [e.g.,
Williams and Yair, 2006], although the altitude of the incloud flash component may play a role as well [Lyons
et al., 2003; Lang et al., 2010, 2011]. Note that a few exceptions to the occurrence of positive sprites over the stratiform
region of MCSs exist [e.g., Lyons et al., 2008]. However,
even in these cases, a common motif is the transition of active
convection toward a more stratiform precipitation structure,
providing increased lateral areas of space charge.
[5] Based on the observations of positive sprite meteorology, one might expect that negative sprites, though rarer,
would feature the same basic characteristics. Indeed, the initial observations of confirmed negative sprites suggested
their occurrence within the stratiform regions of large
MCSs, though the meteorology of these sprites was not particularly well documented [Barrington-Leigh et al., 1999,
2001; Taylor et al., 2008].
[6] However, there are reasons to doubt that this would be
the most common phenomenology. Lu et al. [2012] showed
lightning mapping and other data for 85 negative CGs with
high (> 200 C km) impulse charge moment change values
(iCMC; i.e., the CMC produced within the first 2 ms of the
return stroke). They also presented observations of four
strokes that produced confirmed negative sprites. These
sprites were observed over active convective regions.
Lightning mapping data, which were available for one of
the sprites, suggested that the parent discharge consisted of
a hybrid intracloud (IC)-CG discharge, which also involved
the upper positive charge layer in the normal polarity thunderstorm tripole, as opposed to just occurring between the
midlevel negative and lower positive charge like typical
negative CGs [Lu et al., 2012]. This hybrid IC/CG activity
also was commonly seen in negative CGs that produced the
largest iCMCs (~200 C km or more) in the Lu et al. [2012]
data set. Such hybrid IC/CGs would be more likely to
produce sprites than typical negative CGs because of the
greater charge moment changes. Note that because negative
CGs do not typically feature long continuing currents, their
iCMC values are very similar to their total (impulse plus continuing current) charge moment changes [Lu et al., 2012].
[7] Thus, a reasonable hypothesis for the meteorology of
negative sprites—or of negative CGs with extremely large
charge moment changes that very likely should produce
sprites even if there is no camera to observe them—is that
2.
Data and Methodology
[8] Radar and lightning data for all cases were analyzed
using locally developed software programmed in the
Interactive Data Language. In this section, discussion of each
data source is divided along the different platforms providing
those data.
2.1. Charge Moment Change Network
[9] The Charge Moment Change Network (CMCN) consists
of two extremely low frequency sensors: one near Durham,
NC, USA and one at Yucca Ridge near Fort Collins, CO,
USA [Cummer et al., 2013]. These sensors provide real-time
estimates of iCMC values for CG lightning strokes that occur
within most of the contiguous United States. The National
Lightning Detection Network (NLDN) is used to geolocate
strokes with reported iCMC values and also provides peak
current information, following the methodology of Cummer
et al. [2013].
[10] Since photographic or video confirmation of negative
sprites is rare, it is attractive to exploit the known relationship
between CMC and sprite occurrence in order to develop a
larger data set. However, it is difficult to establish a fixed
criterion for this purpose. Charge moment changes for past
observations of negative sprite parents range from 450 to
1550 C km [Barrington-Leigh et al., 1999, 2001; Taylor
et al., 2008; Soula et al., 2009; Lu et al., 2012; Li et al.,
2012]. However, recent theoretical work has established that
negative CGs with CMCs as low as 300 C km could produce
sprites [Qin et al., 2012]. One would expect that the probability of sprite occurrence would increase as CMC increases,
although mesospheric and ionospheric conditions should be
important as well [Huang et al., 1999; Hu et al., 2002;
Cummer and Lyons, 2005; Li et al., 2008; Lyons et al.,
2009; Qin et al., 2012]. Thus, to ensure the highest probability
of a negative sprite in the CMCN data, the CMC threshold
should be set as large as possible. On the other hand, the value
of utilizing the CMCN data set is to drastically increase the
available number of cases, so setting the threshold too high
will prevent that.
[11] A peak current (Ipk) threshold is difficult to set as well,
though all past observations of confirmed negative sprite
parents with the exception of one (Ipk ~ 50 kA) [Soula et al.,
2009] featured Ipk values of 90 kA or greater [BarringtonLeigh et al., 1999, 2001; Taylor et al., 2008; Lu et al.,
2012; Li et al., 2012]. In fact, Ipk is likely not a primary
control on sprite occurrence [Lyons, 2006], although a low
Ipk for a corresponding large iCMC value (which should be
driven by a high Ipk since iCMC by definition does not
include much continuing current) could be indicative of a
potential data quality problem [Cummer et al., 2013]. Thus,
a high Ipk threshold would be preferable to none at all, simply
to minimize the chance of data problems.
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LANG ET AL.: LARGE CHARGE MOMENT NEGATIVE LIGHTNING
[16] The radar characteristics of storms producing negative
sprites or negative sprite-class lightning were examined. The
time interval of the radar volume including the time of the
sprite-class lightning, as well as the two leading and two
following volumes (i.e., a 25 min period inclusive of the
discharge in question), was included in this analysis. The
vertical behavior of the 10 and 30 dBZ echo contours was
examined within 15 km of the discharge in question in all
volumes. These are common reflectivity thresholds used in
a variety of studies of precipitation systems, and their behavior will be sufficient to diagnose gross characteristics of the
storms of interest. In addition, two-dimensional composites
of maximum reflectivity in the column were developed for
the volume containing the sprite-class lightning, and the contiguous 30 dBZ areas of the parent storms were calculated
from these. Except in limited circumstances, the 10 dBZ area
was not examined because some of the precipitation systems
were so large that it would have required stitching more than
two NMQ tiles together to properly compute 10 dBZ
statistics, a process that consumed excessive resources for
comparatively little additional information. In those cases,
the 30 dBZ contour was more than adequate for indicating
the massive size of the precipitation systems.
[12] Thus, for the purposes of this study, negative CGs
with both peak currents of 100 kA or greater and iCMC
values of 800 C km or greater (<< 0.01% of all negative
CGs with a computed iCMC in the data set) were considered
“sprite-class” lightning, that is, lightning powerful enough to
have likely produced a negative sprite. The 800 C km value is
much larger than the theoretical minimum CMC [Qin et al.,
2012] and is larger than that for many confirmed negative
sprite parents [Taylor et al., 2008; Lu et al., 2012; Li et al.,
2012], but is close to the median of past studies of negative
sprites ( 819 C km). Thus, to the extent that the limited past
observations allow, one can be confident that negative CGs
of this magnitude are very likely to produce sprites. For
example, Lyons et al. [2009] found that positive CGs with
iCMC values greater than 300 C km were 75–80% likely to
produce a sprite. This study’s criterion, though for negative
CGs, is nearly 3 times that number.
[13] The Ipk criterion ( 100 kA) also matches the pattern
of setting a threshold that is near the median observed in
the literature for sprite parents ( 126 kA) and that is high
enough to avoid data quality issues, but not so high as to
create a data set that is too small. Overall, the two criteria
provided a reasonable number of cases for an initial analysis
of negative sprite meteorology (38 sprite-class events which
also featured available radar data).
[14] It is certainly possible that these fixed thresholds—
though consistent with the literature—could exclude potential sprite-producing CGs or for that matter include powerful
discharges that did not actually produce a sprite. Thus, to
ensure that any meteorological inferences based on this data
set are not grossly at odds with those for confirmed negative
sprites, analysis of the meteorology of three negative sprite
parents has been performed. The lightning data for these
sprite parents were analyzed by Li et al. [2012], and their
iCMC values were computed in postprocessing—following
the methodology of Cummer and Inan [2000]—that is more
robust than the real-time CMCN computations. For these
particular events, the postprocessing reduced iCMC
estimates by approximately one third on average, compared
to the real-time values. This is in line with the typical factor
of 1.5 of random error that exists in real-time iCMC estimates
[Cummer et al., 2013].
2.3. NLDN
[17] The performance and data characteristics of the NLDN
are well known to the atmospheric electricity community
[Cummins et al., 1998; Biagi et al., 2007; Cummins and
Murphy, 2009] and will not be reviewed here. Flash-level data
were analyzed to examine the CG characteristics of storms
producing negative sprites or negative sprite-class lightning.
CGs were examined within 15 km of the sprite-class lightning
for the 25 min period inclusive of the discharge. Positive CGs
were not considered if their peak currents did not exceed
10 kA [Cummins et al., 1998]. NLDN-detected IC discharges
of either polarity were considered CGs if their peak currents
exceeded 25 kA (R. Holle, 2009, http://www.srh.noaa.gov/
media/abq/sswhm/Holle_sw_hydromet_09.pdf). IC lightning
flashes were not examined in this study due to their low and
potentially regionally dependent detection efficiency by
NLDN [Cummins and Murphy, 2009].
2.4. LMA
[18] The North Alabama Lightning Mapping Array
(NALMA) [Goodman et al., 2005] provides three-dimensional
mapping of very high frequency (VHF) radiation from
lightning. These data were used to examine the structure of
one sprite-class discharge (on 26 March 2011) that occurred
within range of the NALMA. All of the other sprite-class
lightning examined in this study occurred too far from any
of the various LMA networks that populate the United States,
or the LMA data were already examined in other studies
[Lu et al., 2012].
2.2. NMQ National Radar Mosaics
[15] NOAA has developed the National Mosaic and MultiSensor Quantitative Precipitation Estimation (NMQ) system
in order to improve precipitation estimates and achieve other
related goals [Zhang et al., 2011]. A major component of this
system is the development of three-dimensional radar
mosaics covering the entire contiguous United States.
These mosaics, available starting in 2009, are developed
from the nationwide network of S-band Doppler weather
radars, following a methodology described by Zhang et al.
[2011]. Radar reflectivity is provided every 5 min on a
0.01° latitude/longitude grid, with a vertical coordinate that
ranges from 0.25 km MSL to 18 km (vertical spacing is
0.25 km near the surface gradually stretching to 2 km aloft).
Due to file size limitations, the national mosaics are broken
into eight different tiles. When necessary (e.g., a storm of
interest straddled the border between two tiles), these tiles
were stitched together in the analysis software before
examination was performed.
3.
Results
3.1. General Observations
[19] CMCN data were examined for all negative CG
strokes that met the 800 C km and 100 kA sprite-class threshold over the period from August 2007 to July 2011. The
results are plotted in Figure 1. A total of 92 discharges meeting these criteria were detected and geolocated. This number
3
LANG ET AL.: LARGE CHARGE MOMENT NEGATIVE LIGHTNING
UTC (roughly 02 local time in the central United States).
The 92 strokes were distributed roughly evenly throughout
March through November, averaging ~10 strokes per month
during that period, but none occurred during December
through February.
[22] From these 92 discharges, 38 that occurred during
2009–2011 (when NMQ radar mosaics were available) were
selected for further analysis, based on their geographic
proximity to radar coverage. This naturally excluded some
discharges from this time period that occurred over the
Atlantic Ocean, Mexico, the Caribbean Sea, and the Gulf of
Mexico. This limits the applicability of this study’s results
to precipitation systems occurring mainly over land
(although, as will be seen, some storms occurred in coastal
regions), despite the fact that powerful negative CGs are
disproportionately favored over salt water [Lyons et al.,
1998]. An additional three discharges were added to this
database, for a grand total of 41 negative CGs. The three
additions were negative CGs that occurred in 2010 and
2011 and did not meet the iCMC criteria (values ranged from
450 to 710 C km), but did produce confirmed negative sprites
as reported by Lu et al. [2012] and Li et al. [2012].
[23] The radar data for the storms producing these 41
discharges, which occurred over 23 different days, were
examined in detail. A representative example of a negative
sprite-class storm is shown in Figure 2. This storm was an
intense, multicellular system that occurred over northeast
Texas on 3 May 2009. The sprite-class discharge occurred
within the convection, which itself was producing mostly
negative CGs (62.5% of 104 CGs during the 25 min analysis
period). The 30 dBZ contour extended to an average
maximum height of 16 km MSL, and the areal coverage of
contiguous 30 dBZ echo in this storm was nearly 5500 km2.
[24] Overall statistics for the 41 cases are shown in Table 1.
The results suggest that most sprite-class/parent negative
CGs occurred in or near large (at least MCS-scale), intense,
multicellular convection that was mainly producing negative
CGs. Indeed, all but three events occurred within 15 km of a
convective core that featured 30 dBZ reaching to at least
10 km MSL. The remainder occurred within the stratiform
regions of MCSs. Trends in 30 dBZ echo heights and
volumes during this period (not shown) were mixed, with
no clear trend toward increasing or decreasing intensity at
the time of sprite-class stroke production. Given the
behavior, the most general statement that can be made is that
the precipitating structures near the sprite-class CG were mature. All but six events produced greater than 50% negative
CG lightning during their respective 25 min analysis periods,
although overall CG flash rates tended to be low (1–2 min 1
on average).
[25] Figure 3 shows a distribution of contiguous 30 dBZ
echo areas for all 41 events. Only seven were smaller than
2000 km2 (the smallest observed system was 616 km2), while
a full third of the data set occurred in massive systems totaling 50,000 km2 or more in area. For context, the areas of
those large systems were comparable to the commonly
accepted definition of mesoscale convective complexes
[Maddox, 1980]. However, the present study used a much
more restrictive criterion, namely, contiguous 30 dBZ
radar echo as opposed to satellite cold cloud structure.
As another point of comparison, the prolific positive
sprite-producing MCS on 20 June 2007, studied by
Figure 1. Map of 92 detected negative CGs with iCMC
values greater than 800 C km and peak currents greater than
100 kA, for the period August 2007 through July 2011 (blue
triangles). The asterisks denote the locations of the two
CMCN sensors, with their nominal 2000 km detection ranges
denoted by the dashed circles.
was less than 5% of the number of positive CGs (1875)
meeting the same criteria. This supports the findings of other
studies that large-CMC negative discharges are much rarer
than their positive counterparts [Cummer and Lyons, 2005;
Williams et al., 2007; Cummer et al., 2013]. Most of the
discharges were loosely clustered around the lower
Mississippi River valley and Gulf of Mexico, as well as the
Gulf Stream off the eastern coast of the United States.
These regions are roughly consistent with those featured in
the stroke density maps for weaker ( 200 and 600 C km
thresholds) negative discharges shown by Cummer et al.
[2013], which bolsters confidence that the present study’s
data set can be considered an approximately representative
sample of a negative CG data set with a more relaxed
iCMC criterion, at least from the perspective of regional
climatology. For every 100 C km that the iCMC threshold
is relaxed, the available sprite-class data set for this same
time period approximately doubles in size (203 strokes for
700 C km threshold, 519 strokes for 600 C km, 1271 strokes
for 500 C km, 3254 strokes for 400 C km, and 8755 strokes
for 300 C km).
[20] An interesting question to ask is whether the 92 spriteclass negative CGs commonly occurred with other large-iCMC
strokes, positive or negative. Fifty-six of these sprite-class
negatives occurred within 15 min and 100 km of at least one
other powerful negative CG (threshold: iCMC < 300 C km,
Ipk < 50 kA). The median number of companion powerful
negatives was 1, and the maximum was 12, for each of the 92
sprite-class events. However, it was rare for powerful positive
CGs to occur in concert with the sprite-class negatives. Only
one sprite-class negative CG occurred within 15 min and
100 km of a positive CG with the same characteristics
(iCMC > 800 C km, Ipk > 100 kA), and only 9 of 92 occurred
within the same distance and time of positive CGs with relaxed
criteria (iCMC > 300 C km, Ipk > 50 kA).
[21] Most (79) of the 92 sprite-class negative CGs occurred
during the 00–12 UTC time period, which in the United
States corresponds to evening, nighttime, and early morning
hours. The mode of the diurnal distribution occurred at 07
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LANG ET AL.: LARGE CHARGE MOMENT NEGATIVE LIGHTNING
Figure 2. (a) Plan view of composite radar reflectivity at 0100 UTC on 3 May 2009 for a storm that
produced a sprite-class negative CG (large triangle) in eastern Texas during the radar volume. NLDNdetected positive (X) and negative (small triangle) CGs during the volume are also shown. The dotted
grid lines are spaced 0.2° in latitude/longitude, and the dashed line denotes the vertical cross section in
Figure 2b. (b) Vertical cross section through the same longitude as the sprite-class negative CG. The
dashed line denotes the latitude of the CG. The subplot title gives iCMC and peak current information,
respectively, for the discharge.
within 15 km of a deep convective core and thus is not considered a stratiform-produced stroke.
[27] The other example was a large mesoscale system that
occurred on 9 March 2011 and produced seven sprite-class
negative CGs over a ~16 h period (Figure 5). When it first
started producing sprite-class negatives, the system consisted
of a large number of distinct multicellular convective clusters
(Figures 5a–5c). The sprite-class negatives were mostly split
between separate, mature clusters at a few different times
during this day. As the system moved eastward over the
Gulf coast states—ahead of an eastward moving cold
front—it gradually organized into a classic leading-line,
trailing-stratiform MCS. Distinct from the other events
on this day, the last sprite-class negative CG occurred in
the stratiform region of the MCS, unfortunately too far
Lang et al. [2010], featured peak radar-defined convective
areas near 50,000 km2. Also, recall that the typical precipitation system does not produce positive sprites until it
reaches 20,000–25,000 km2 in total radar echo coverage
(not just 30 dBZ, as in this study) [Lyons, 1996, 2006].
A common motif in the data set was the presence of large,
quasi-steady (i.e., not rapidly evolving) precipitation systems aligned along fronts and other boundaries.
3.2. Systems That Produced Several Sprite-Class
Negative CGs
[26] The typical pattern was for a storm system to produce
only one to three negative sprite-class discharges during its
lifetime. As noted before, these generally were produced near
mature, deep, and intense convective cores. However, there
were a couple of notable storms that produced more than
three sprite-class strokes. On 5 October 2010, a relatively
small linear MCS along the Gulf coast produced seven
sprite-class negative CGs over a ~6.5 h period (Figure 4).
The 30 dBZ composite areas of the convective elements producing these sprite-class strokes ranged between 1500 and
3000 km2. However, a less restrictive threshold, such as a
10 dBZ contiguous area, found total storm areas ranging
from ~20,000 km2 to ~35,000 km2, in line with Lyons
[1996, 2006]. The MCS was slow moving, remaining mostly
over east Texas and Louisiana for the majority of this time,
and was aligned roughly parallel to the coast, likely along a
sea-breeze front (Figure 4). Though it struck ground within
a weak echo, the sprite-class negative in Figure 4a occurred
Table 1. Characteristics of the 41 Negative Sprite-Producing or
Sprite-Class Precipitation Systems Observed in This Study
Minimum Median Maximum
1
CG flash rate (min ) within 15 km
% Negative CGs within 15 km
10 dBZ Max height (km MSL)
within 15 km
30 dBZ Max height (km MSL)
within 15 km
2
30 dBZ Contiguous area (km )
Maximum reflectivity within 15 km (dBZ)
Maximum reflectivity in feature (dBZ)
5
0.1
21.0
13.8
1.6
69.2
17.6
4.5
14.2
616
39.5
43.5
6.1
100
18.0
16.4
3
5
6.73 × 10 1.99 × 10
53.0
63.0
58.5
67.0
LANG ET AL.: LARGE CHARGE MOMENT NEGATIVE LIGHTNING
Figure 3. Distribution of 30 dBZ composite reflectivity contiguous areas for negative sprite-class/parent
precipitation systems in this study. In order to avoid distorting the presentation of the smaller systems, the
long tail of systems with greater than 50,000 km2 area is lumped into a single category.
3.3. Lightning Mapping Analysis
[28] One negative sprite-class storm, on 26 March 2011,
was close enough to the NALMA for useful analysis. The
discharge in question (peak current
145 kA, iCMC
974 C km) occurred near a large cluster of convective cells
from the NALMA for useful analysis (Figure 5d). This
stratiform region was intense, however, with 30 dBZ
reaching a maximum altitude of 4.5 km on average, and
the maximum reflectivity within 15 km of the stroke was
47.5 dBZ.
Figure 4. Composite reflectivity at (a) 0540, (b) 0715, (c) 0740, and (d) 0800 UTC on 5 October 2009,
showing an MCS along the Gulf coast in southeast Texas and central Louisiana. Also shown are the
sprite-class negative CGs (triangles) that occurred during each volume: iCMC = 1117 C km,
Ipk = 207 kA (Figure 4a); iCMC = 1265 C km, Ipk = 198 kA (Figure 4b); iCMC = 868 C km,
Ipk = 140 kA (Figure 4c); and iCMC = 995 C km, Ipk = 193 kA (Figure 4d). Dashed grid lines are
spaced 0.5° in latitude/longitude.
6
LANG ET AL.: LARGE CHARGE MOMENT NEGATIVE LIGHTNING
Figure 5. Maps of composite reflectivity at (a) 0100, (b) 0515, (c) 0750, and (d) 1715 UTC for multiple
precipitation systems on 9 March 2011. Also shown are the sprite-class negative CGs (triangles) that occurred during each volume: iCMC = 971 C km, Ipk = 160 kA (Figure 5a); iCMC = 1112 C km,
Ipk = 201 kA (Figure 5b); iCMC = 896 C km, Ipk = 143 kA (Figure 5c); and iCMC = 817 C km,
Ipk = 102 kA (Figure 5d). Dashed grid lines are spaced 1° in latitude/longitude.
Figure 6. (a) Time-height plot of VHF sources (asterisks) detected by the NALMA for a sprite-class negative CG (iCMC 974 C km, peak current 145 kA) occurring at 10:43:02 UTC on 26 March 2011. The
diamond near 10 km is the initial VHF source point, and the triangle is the negative CG. (b) Plan view of the
discharge (asterisks now red, all other symbols the same) overlaid on top of composite reflectivity from the
10:40 UTC radar mosaic. The dashed grid lines are spaced 0.1° in latitude/longitude.
7
LANG ET AL.: LARGE CHARGE MOMENT NEGATIVE LIGHTNING
Figure 7. (a) Map of composite reflectivity (0645 UTC) for a precipitation system that produced a confirmed negative sprite parent CG (triangle; iCMC 710 C km, peak current 127 kA) over eastern
Oklahoma during the radar volume on 9 September 2009. (b) Same as Figure 7a but for 0815 UTC on
25 August 2011 (iCMC 450 C km, peak current 160 kA) over eastern Texas. Dashed grid lines are
spaced 1° in latitude/longitude.
iCMC negative CGs in general, the flash appeared to be a
hybrid IC/CG discharge, which started between the midlevel
negative and upper-level positive charge regions and then
came to ground late in the flash’s lifetime. This behavior
around 1043 UTC (contiguous 30 dBZ area of the spriteclass storm was 15,330 km2), and the LMA showed initiation
and termination of the discharge near the convection
(Figure 6). Similar to what Lu et al. [2012] found for large-
Figure 8. Map of composite reflectivity (0430 UTC) for a precipitation system that produced a confirmed
negative sprite parent CG (triangle; iCMC 560 C km, peak current 102 kA) over eastern Colorado during the radar volume on 29 July 2011. Dashed grid lines are spaced 1° in latitude/longitude.
8
LANG ET AL.: LARGE CHARGE MOMENT NEGATIVE LIGHTNING
by Lang and Rutledge [2011], the 30 dBZ contour is commonly used in radar studies of storm electrification.
Moreover, the results of Lu et al. [2012] suggest that 15 km
is an approximate minimum horizontal distance that the incloud components of high-iCMC negative flashes can travel
between initiation and ground strike. Regardless, the focus
of this study was on gaining a qualitative sense of the meteorology of negative sprite-class lightning, and sensitivity studies (not shown) demonstrated that adjusting these thresholds
did not affect its main conclusions.
[32] These conclusions are the following:
[33] 1. Negative sprite-parent and negative sprite-class lightning mainly occurred in or near intense, deep convection.
[34] 2. The systems producing these discharges were
frequently large—at least MCS scale, thousands if not tens
of thousands of square kilometers in size. This is in basic
agreement with past studies of negative sprite-producing
storms [Barrington-Leigh et al., 1999, 2001; Taylor et al.,
2008; Soula et al., 2009]. Large, long-lived, and quasi-steady
systems organized by fronts and other boundaries could
produce multiple sprite-class strokes over several hours.
[35] 3. The systems producing these discharges also
produced mainly negative as opposed to positive CG
lightning near the locations of the sprite-class discharges, although overall CG flash rates were low (1–2 min 1,
on average).
[36] This latter conclusion suggests that negative sprites
would tend to occur over normal polarity thunderstorms,
albeit ones that are very large in area to be able to support
the large observed charge moment changes. There were
notable exceptions to all these observations, for example,
the three systems that produced stratiform sprite-class or
sprite-parent lightning. In these cases, the stratiform regions
also were mature and intense, with maximum reflectivities
near the sprite-class/parent strokes ranging from 39.5 to
47.5 dBZ and maximum heights of the 30 dBZ contour
ranging from 4.5 to 6.7 km MSL.
[37] The general pattern that was observed for negative
sprite parents or sprite-class lightning suggests an interesting
contrast to the meteorology of positive sprite-parent lightning.
Positive sprite parents normally occur within the stratiform
regions of MCSs [Boccippio et al., 1995; Lyons, 1996, 2006;
Lyons et al., 2003; Williams and Yair, 2006; Lyons et al.,
2009], with some notable exceptions [e.g., Lyons et al.,
2008]. Thus, while the sizes of precipitation systems producing negative or positive sprite-parent/class lightning appear
to be similar, there is a distinct difference in where the opposite
polarity discharges typically occur, one that mirrors the typical
pattern of negative and positive CG occurrence in MCSs [e.g.,
Rutledge and MacGorman, 1988; Rutledge et al., 1990].
[38] However, this does not necessarily mean that the same
precipitation system would produce negative and positive
sprites concurrently, due to the observed lack of highiCMC positive and negative CG strokes occurring close in
space and time to one another. This is consistent with the
observed regional offset in the United States between the
occurrence of powerful positive strokes (favored mainly over
the northern Great Plains) and powerful negative ones
(favored mainly over the southeast and nearby oceanic
regions) [Lyons et al., 1998; Williams et al., 2005; Cummer
et al., 2013]. Why this offset exists, and to what extent it
impacts individual storms, is a topic of future research. Of
(upper-level intracloud activity preceding an eventual
negative leader to ground) is qualitatively similar to what
has been noted for so-called “bolt-from-the-blue” (BFB)
flashes [Krehbiel et al., 2008; Lu et al., 2012], suggesting that
BFBs may disproportionately feature large CMCs and
therefore greater halo and/or sprite potential than normal
negative CGs. Note that this flash occurred ~120 km from
the NALMA network center, so three-dimensional mapping
and source detection was not optimal, preventing good resolution of the midlevel negative and low-level positive charge
regions. This storm produced 68.3% negative CG lightning
flashes (out of 41 total CGs), and its 30 dBZ echo reached
13 km MSL on average during the 25 min analysis period
encompassing the sprite-class discharge. The NALMA indicated a decreasing trend in total lightning flash rate in the cell
that produced the sprite-class stroke, with a flash rate of
10 min 1 at 1039 UTC declining to only 3 min 1 (including
the sprite-class one) during 1043 UTC.
3.4. Confirmed Negative Sprite-Producing Systems
[29] Three of the events in this study were observed to
produce confirmed negative sprites. These occurred on 9
September 2010, 29 July 2011, and 25 August 2011. The
detailed analysis of the sprite-parent flashes can be found in
Lu et al. [2012] and Li et al. [2012] and will not be reviewed
here. Two of the events (9 September 2010 and 25 August
2011) occurred within the convective regions of multicellular
systems (Figure 7), similar to the overall results with other
sprite-class lightning in this study, while the third (29 July
2011) occurred over the stratiform region of an irregularly
shaped MCS (Figure 8). The 9 September 2010 storm
included the remnants of Tropical Storm Hermine and
40–45 min later produced two gigantic jets [Meyer et al.,
2013]. The contiguous 30 dBZ areas for the convective
negative sprite events were 52,181 km2 and 3072 km2,
respectively. For the 25 min analysis periods encompassing
the sprite parents, the first storm (9 September 2010) featured
30 dBZ reaching 12.8 km MSL on average and produced
67% negative CG lightning (out of 24 CGs total), while the
second (25 August 2011) featured 30 dBZ reaching 14.6 km
MSL on average and produced 100% negative CG lightning
(19 CGs). By contrast, the stratiform event had 30 dBZ
reaching 6.7 km MSL on average, and there were only two
CGs total (100% negative). Nevertheless, the 30 dBZ contiguous area (which included both stratiform and convective
echo) was just under 20,000 km2, and the maximum reflectivity within 15 km of the sprite-parent stroke was 45.5 dBZ.
[30] Overall, the characteristics of the confirmed negative
sprite-producing precipitation systems were similar to those
observed for the sprite-class storms. The parent strokes
occurred either within intense convection contained within
large mesoscale systems, or within a well-developed stratiform region, and these storm areas produced low rates
(~1 min 1 or less) of mainly negative CG lightning.
4.
Discussion and Conclusions
[31] This study featured the use of several numerical
thresholds, such as 10 and 30 dBZ echo heights and/or areas,
as well as 15 km ranges. These thresholds, though arbitrary,
were informed by common thresholds for convective studies
used by the scientific community. For example, as reviewed
9
LANG ET AL.: LARGE CHARGE MOMENT NEGATIVE LIGHTNING
sprites with high-speed video camera observations as well
as adding more cases of negative sprite-class lightning from
upcoming and past convective seasons. The results of the
present study suggest that video observations over the convective regions of large, quasi-steady precipitation systems
—particularly over the southern and southeastern United
States—may be fruitful in this regard. Over the long term, a
detailed database of the characteristics of storms producing
this lightning can be developed, and statistically based analysis of these systems can be performed. The development
of the national radar mosaics, along with the proliferation
of optical and radio-frequency observing networks for sprites
and their parent lightning, will greatly facilitate future
research into sprite meteorology.
particular interest is the fact that negative and positive sprites
have been observed in close proximity to one another in other
regions of the world [Barrington-Leigh et al., 1999; Taylor
et al., 2008; Soula et al., 2009].
[39] The limited LMA analysis in this study, coupled with
the detailed analysis of negative high-iCMC flashes and a
negative sprite parent by Lu et al. [2012], suggests that incloud components of the negative lightning remain mostly
near convection and consist of a hybrid-IC/negative-CG
discharge that involves the upper positive charge region
during the flash—characteristics that are qualitatively similar
to BFB discharges. This is an interesting point of comparison
between high-iCMC negative lightning and positive sprite
parents. A typical positive parent often initiates in convection
and travels into the stratiform region along a downward
sloping upper pathway (which can reach as low as the
melting level far into the stratiform region) before coming
to ground [Lang et al., 2010, 2011], while negative highiCMC strokes may involve the upper positive charge only
within the convection proper. In this hypothesized model,
lower positive charge in the normal tripole structure of the
convective region helps to eventually guide the upper-level
intracloud activity to ground as a negative CG. However, if
the upper-level lightning travels into the stratiform region,
then only a sunken positive dipole may exist, and thus, the
preferred path to ground is as a positive CG.
[40] The common involvement of the upper positive
charge in sprite-parent or sprite-class lightning, regardless
of the polarity of the final CG (and sprite), is a notable result
coming from a synthesis of this study with the results of
others [Lang et al., 2010, 2011; Lu et al., 2012]. If largeiCMC positive and negative strokes (e.g., sprite parents)
compete for available upper positive charge in convection,
then that may help explain their observed lack of coincident
and collocated occurrence in the United States.
[41] It is notable that negative CG flash rates were typically
low near the sprite-class negative strokes and that for the one
analyzed LMA case (26 March 2011), total flash rates in the
parent cell were low and declining. This is consistent with the
relatively low CG rates observed during the time of the negative sprite reported by Soula et al. [2009]. These low flash rates
may allow the buildup of large amounts of negative charge that
can be subsequently neutralized by a single stroke, leading to a
large peak current and iCMC (T. Chronis et al., New evidence
on the diurnal variation of peak current in global CG lightning,
submitted to Bulletin of the American Meteorological Society,
2013). The large contiguous regions of deep convection in
many of the observed sprite-class systems also should assist
with copious charge production over a broad area.
[42] A limitation of the present work is that it lacks confirmation of sprite occurrence for most of the examined cases. It
also could not focus on the radar structure of sprite-class
storms over the Gulf Stream and other oceanic regions.
Moreover, it is well known that negative halos are much
more common than negative sprites [Bering et al., 2004;
Frey et al., 2007; Williams et al., 2007, 2012; Newsome
and Inan, 2010]. This study was not focused on these particular TLEs, but they are related to sprites and can occur over
negative CGs with similar CMCs to the thresholds used in
this study [Williams et al., 2012].
[43] In addition to addressing the above concerns, future
work should focus on continued searching for negative
[44] Acknowledgments. Vaisala supplied the NLDN flash-level data
analyzed in this study and also enables the CMCN by providing geolocation
of high-iCMC strokes via a real-time NLDN stroke-level data feed. These
data were absolutely critical to this study, and the authors offer their sincerest
thanks to Vaisala for providing them. Katherine Willingham provided the
NMQ radar mosaic data on behalf of NOAA. Bill McCaul of Universities
Space Research Association provided the NALMA data. Paul Hein of
Colorado State University assisted with the data analysis. The authors are extremely grateful to all of these people and their agencies for their gracious
help in facilitating this research. The authors also thank the journal editor
and reviewers for their assistance with publishing this study. This research
was funded by the Defense Advanced Research Projects Agency under the
Nimbus program, as well as the National Science Foundation under
grant AGS-1010G6S7.
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